1. Field of the Invention
The present invention relates to an electrochemical gas sensor and a mounting structure therefor.
2. Description of the Related Art
In a proton-conductive gas sensor, water vapor is supplied from a water reservoir to a proton-conducting membrane. For example, according to U.S. Pat. No. 6,896,781, a proton-conducting membrane is provided with a sensing electrode on one surface and a counter electrode on the opposite surface, and each of the sensing electrode and the counter electrode is covered with a hydrophilic conductive membrane, which is hydrophobic, gas-permeable, and conductive. Here, an assembly of a proton-conducting membrane, a sensing electrode and a counter electrode is called an “MEA” (membrane electrode assembly). According to U.S. Pat. No. 6,896,781, a water reservoir is provided on the counter electrode side and an atmosphere to be detected is supplied from the sensing electrode side, thus preventing water in the water reservoir from overflowing to the proton-conducting membrane due to the presence of the hydrophilic conductive membrane.
The Applicant of the present application has proposed to provide a gas sensor using a proton conductor with a metal plate having a diffusion control hole formed therein on the sensing electrode side of the gas sensor, thus controlling gas diffusion to the sensing electrode (see, for example, U.S. Pat. No. 7,393,505). The Applicant of the present application has also proposed that a separator, instead of a proton conductor, may retain an electrolyte (see, for example, U.S. Pat. No. 7,381,314). Moreover, the Applicant of the present application has proposed omitting the water reservoir, measuring the impedance of the proton-conducting membrane or the separator, and correcting a reduction in output of the gas sensor in a dry atmosphere by the impedance of the proton-conducting membrane or the like (see, for example, WO 2011/96106A). As a result of investigating how to increase the gas sensitivity of an electrochemical gas sensor having no water reservoir, the present inventors have developed and achieved the preferred embodiments of the present invention.